Brenneria (Erwinia) quercina

Bark canker and drippy nut of Quercus

Why: In 1967, a new bacterial disease of oak caused by Erwinia quercina (later reclassified as Brenneria quercina) was reported in California (US) and called drippy nut, due to significant bacterial ooze observed on acorns. Apparently, this disease was no longer observed in USA nor reported from other countries until 1992, when the bacterium was found on forest oaks in Spain, causing slightly different types of damage (ie. bark cankers and bacterial ooze on leaf buds).

Where:EPPO region: Spain (near Madrid and Segovia; first isolated in 1992 on Q. ilex; in 2001, also reported in Comunidad Valenciana).North America: USA (California, first description made in 1967 and apparently no further reports were made since then).

On which plants:Quercus spp. (Q. agrifolia and Q. wislizenii in California, Q. ilex and Q. pyrenaica in Spain). More data is needed on the susceptibility of other European Quercus species.

Damage: In California, the disease was described as ‘drippy nut’. The first visible symptoms were darkening and oozing at insect punctures in the acorns. Bacterial ooze was observed at the base of the nut. Rotting of the nuts was also observed. After the nut had fallen, in some cases the acorn cup produced bacterial ooze. The disease occurred in later summer when day temperatures were hot (average around 29°C).
In Spain the disease presented some additional symptoms. The first symptoms were bark cankers, although the ‘drippy nut’ symptom was observed in later surveys. A frequent symptom was the presence of irregular, longitudinal cankers of a few centimetres in size on the trunk and branches (reaching in some cases up to 20 cm). These cankers, variable in depth, showing necrosis of the affected tissues and copious exudations, usually appeared on the bark surface of the lower trunk. Necrotic lesions extended to inner bark. Oak trees seriously affected by bark cankers were usually mature (more than 20 years old), showing a progressive loss of vigour, foliage reduction and early leaf senescence. Exudates were frequently observed in growing acorns. Copious, sticky, honey-like sap appeared under the acorn cup and caused severe fruit drop. In many cases, acorns rotted. Exudates from leaf buds were also observed in some Q. pyrenaica trees, which were not described in California. Surveys done in forests near Madrid since 1996 have shown that symptoms were very widespread and that approximately 30 to 40 % of the acorns were affected. Acorns are a valuable source for wildlife, as well as for feeding pigs.

Dissemination: It is suspected that the bacteria enter through wounds, but it is not known whether the lesions observed in natural conditions are produced by or just used by the bacterium to enter into the plant. In California, it has been suggested that the bacterium entered the acorns through holes made by insects, especially by Cynipidae. Water most probably plays a role in disseminating the bacterium.

Pathway: Plants for planting of Quercus, seeds ?

Possible risks: Oaks are important forest and amenity trees. No control methods are available against B. quercina. Although more details are needed on the host range, biology, geographical distribution, epidemiology, the observations made in Spain suggested that B. quercina could be damaging to other oak species elsewhere in the EPPO region. The possible role of B. quercina in the oak decline symptom has to be further studied.